The ENDO Study is a collaborative epidemiologic study whose goal is to assess the relation between exposure to environmental chemicals and lifestyle factors that may affect womens reproductive health. Both persistent and non-persistent chemicals are being assessed in relation to the odds of an incident diagnosis of endometriosis. Endometriosis is a condition in which endometrial tissue is found outside the uterus and can be associated with pelvic pain, menstrual changes and infertility. Approximately 11% of women aged 18-44 years may be affected, and many cases may be currently undetected. To date, the cause of endometriosis is unknown and, increasingly, an environmental etiology is suspected for some affected women. A matched cohort design was utilized to establish two cohorts of women - operative and population. 495 women were recruited from 14 participating clinics located in the San Francisco and Salt Lake City areas. This operative cohort was matched on age and residence with a 50-mile catchment area of participating clinical centers to establish a population cohort comprising 131 women who underwent pelvic MRIs to identify endometriosis or its absence. Women participated in baseline interviews and provided blood and urine samples priort to surgery or MRI. Additional operative biospecimens were obtained from the operative cohort when available (i.e., omentum fat, peritoneal fluid, endometrial implants). Serum, fat and urine samples have undergone toxicologic analysis for the quantification of persistent and short lived chemical exposures to assess their relation with endometriosis and, eventually, other gynecologic conditions such as fibroids. A number of persistent and short lived environmental chemicals have been associated with endometriosis. However, the findings vary by method of diagnosing endometriosis (for example by surgical or MRI visualization).